Japanese Unexamined Patent Publication (Kokai) No. Showa 60-11706 discloses a pressurized fluid supply system of the type set forth above. FIG. 1 shows the pressurized fluid supply system disclosed in the above-identified publication. A plurality of pressure compensation valves 3 and 13 are connected in parallel to a discharge line pipe 2 of a hydraulic pump 1. Discharge pipes 4 and 14 of respective pressure compensation valves 3 and 13 are provided with direction control valves 5 and 15. The outlet sides of the direction control valves 5 and 15 are connected to actuators 6 and 16. The pressure compensation valves 3 and 13 are constructed to be biased in a valve opening direction by a pump discharge pressure and outlet pressures of the direction control valves 5 and 15 and to be biased in a valve closing direction by the inlet pressures of the direction control valves and the highest load pressure. With the shown pressurized fluid supply system, it becomes possible to supply pressurized fluid discharged from the pump to respective actuators at a predetermined distribution ratio when a plurality of direction control valves 3 and 13 are operated simultaneously.
However, in the above-mentioned pressurized fluid supply system, it becomes essential to provide a shuttle valve 7 for comparing load pressures of respective actuators and supplying the highest load pressure to the pressure compensation valves. Furthermore, the number of shuttle valves required becomes one less than the number of actuators. Therefore, the number of necessary shuttle valves is inherently increased according to an increasing number of the actuators to be supplied the pressurized fluid thereby resulting in increased costs.
On the other hand, in the pressurized fluid supply system illustrated in FIG. 1, assuming that the load pressure of the actuator 6 is higher than the load pressure of the actuator 16 among the load pressures generated upon actuation of two actuators 6 and 16, a pressure in a passage 8 is introduced into a passage 9 via the shuttle valve 7 as the maximum load pressure. Subsequently, if the load pressure fluctuates and the load pressure of the actuator 16 becomes higher than the load pressure of the actuator 6, the shuttle valve 7 is switched to connect the passage 9 and a passage 18. Upon this switching, due to blow off of the shuttle valve 7, the pressure in the passage 18 drops, and the pressure in the other passage 8 is blocked. As a result, upon switching of the shuttle valve 7, the actuator 16 transitively causes natural drop and the actuator 6 is transitively accelerated.
In order to solve the above-mentioned problem, the applicant has previously proposed a pressurized fluid supply system, in which a plurality of direction control valves 22 are provided in a discharge passage 21 of a hydraulic pump 20, and a pressure compensation valve 25 constituted of a check valve 23 and a pressure reduction valve 24 is provided at the inlet side of each direction control valve, as shown in FIG. 2.
In such pressurized fluid supply system, since a plurality of direction control valves and a plurality of pressure compensation valves are provided, when these are combined, the overall system becomes bulky to require large installation space. Therefore, it becomes difficult to provide installation space for small size construction machines.